The study of either autoimmune and immunodeficiency disorders can provide important and often surprising knowledge about specific genes that control the number and function of white blood cells. Debilitating genetic mutations in CARD11, a key signaling protein in lymphocytes, manifest as severe combined immunodeficiency in mice and humans. On the other hand, spontaneous mutations that inappropriately turn CARD11 signaling on have been described in ~10% of B cell lymphomas that demonstrate continuous activation of NF-?B, a family of proteins that governs multiple genes associated with cell survival, proliferation and differentiation. We recently characterized a new human disorder in which activating, germline- encoded CARD11 mutations cause spleen enlargement and selective expansion of B cells, even though CARD11 signaling is thought to be highly similar in B and T cells. Paradoxically, patients with this disease (termed B cell Expansion with NF-B and T cell Anergy, or BENTA) also show immune system deficits including specific antibody deficiency. The long-term objective of this project is to elucidate how inherited gain-of-function mutations in CARD11 differentially affect B and T cells and ultimately contribute to a novel primary immunodeficiency. Using blood samples from BENTA patients, we will determine if impaired B cell differentiation and antibody secretion is primarily explained by a B cell intrinic signaling defect or a failure of T cells to provide sufficient help to responding B cells. Our analyses will pinpoint specific molecules and biochemical pathways that are disrupted by active CARD11 signaling directly in primary cells from affected patients, an invaluable resource that we have unique access to. Moreover, we will create the first animal model of BENTA disease by introducing the identical, inherited point mutations we discovered in humans into mice. Our proposed experiments with these mice will finally allow us to study perturbations in early lymphocyte development and test B cell responses to defined antigens in vivo in an intact host, which is simply not possible with human patients. This combined approach will significantly revise our understanding of how CARD11 functions as a critical, differential regulator of B versus T cell function. Insights gleaned from a more thorough investigation of this novel PID will hopefully inform new therapeutic approaches for treating BENTA and other immunological disorders that share common features.